This invention relates to support members for superconducting magnet assemblies. More particularly, the invention relates to a support member for suspending a magnet cartridge within a vacuum chamber in a superconductor magnet assembly.
Superconducting magnets typically include a magnet cartridge suspended within an outer vacuum chamber by a plurality of support members, which extend from the outer vacuum chamber to the magnet cartridge. Disposed between the magnet cartridge and the outer vacuum chamber is a radiation shield, through which the support members extend.
To facilitate the superconductivity of the electrical wiring within the magnet cartridge, the magnet cartridge is maintained at a temperature that approaches absolute zero. However, the walls of the outer vacuum chamber are subject to ambient (room) temperature. To maintain this large temperature gradient, the magnet assembly is designed to reduce convection, radiation, and conduction heat transfer between the magnet cartridge and the walls of the outer vacuum chamber.
A reduction in convection heat transfer is accomplished by maintaining a vacuum within the outer vacuum chamber. A reduction in radiation heat transfer is accomplished by the radiation shield, and a reduction of conductive heat transfer is accomplished through the design of the support members.
The support members are subjected to the large temperature gradient—with the end of the support member at the magnet cartridge subjected to temperatures approaching absolute zero, and the end of the support member at the outer vacuum chamber subjected to room temperature. The support members are designed to have very low thermal conductivity and to cater for the effects of differences in the coefficient of thermal expansion of the different materials used in the construction of the magnet and the suspension system. In addition to the thermal stresses, the support members must be designed to withstand forces applied by the magnet. These forces include the weight mass of the magnet, which can be many tons, and the forces induced by the magnet, which can be even greater. The support members must have sufficient stiffness to prevent motion of the magnet when these forces are applied.
Typically, the support members are long, thin rods. Because the rods are long and thin, the heat transfer area is small, which is an advantage in preventing conductive heat transfer. However, these rods provide support in tension only and would buckle if exposed to a compressive load while the forces applied to the support members by the magnet are not constant in direction. Thus, to ensure that the magnet cartridge is supported under the varying forces, the rods are arranged in a matrix surrounding the magnet cartridge.
While such support members are effective in supporting the magnet cartridge, the use of such support members has drawbacks. First, as the number of rods used in the array increases, the conductive heat transfer area also increases. In addition, the number of penetrations through the radiation shield also increases, which decreases the effectiveness of the radiation shield, and increases the labor necessary to seal each of the penetrations from radiation leakage. Second, the rods must be accurately positioned (e.g., in diametrically opposed fashion) and are typically pre-tensioned. The accurate positioning of the rods and the pre-tensioning of the rods add to the cost of manufacturing the magnet assembly.